The present invention relates to very fine-grained particulate material and to methods for producing such very fine-grained particulate material. In preferred aspects, the present invention relates to oxide materials of very fine-grained particulate material and to methods for producing such material. Most suitably, the particulate material has grain sizes in the nanometer scale.
Metal oxides are used in a wide range of applications. For example, metal oxides can be used in:
solid oxide fuel cells (in the cathode, anode, electrolyte and interconnect);
catalytic materials (automobile exhausts, emission control, chemical synthesis, oil refinery, waste management);
magnetic materials;
superconducting ceramics;
optoelectric materials;
sensors (eg gas sensors, fuel control for engines);
structural ceramics (eg artificial joints).
Conventional metal oxides typically have grain sizes that fall within the micrometer range and often are supplied in the form of particles having particle sizes greater than the micrometer range. It is believed that metal oxides that are comprised of nanometer sized grains will have important advantages over conventional metal oxides. These advantages include lower sintering temperatures, potentially very high surface areas, and sometimes improved or unusual physical properties. However, the ability to economically produce useful metal oxide materials with nanometer-sized grains has proven to be a major challenge to materials science. It has proven to be difficult to make such fine-scale metal oxides, particularly multi-component metal oxides, with:
(a) the correct chemical composition;
(b) a uniform distribution of different atomic species;
(c) the correct crystal structure; and
(d) a low cost.
Many important metal oxides have not yet been produced with very fine grains, especially multi-component metal oxides. This is because as the number of different elements in an oxide increases, it becomes more difficult to uniformly disperse the different elements at the ultra-fine scales required for nanometer-sized grains. A literature search conducted by the present inventors has shown that very small grain sizes (less than 20 nm) have only been attained for a limited number of metal oxides. The reported processes used to achieve fine grain size are very expensive, have low yields and can be difficult to scale up. Many of the fine grained materials that have been produced do not display particularly high surface areas, indicating poor packing of grains.
At this stage, it will be realised that particles of material are typically agglomerated of a number of grains. Each grain may be thought of as a region of distinct crystallinity joined to other grains. The grains may have grain boundaries that are adjacent to other grain boundaries. Alternatively, some of the grains may be surrounded by and agglomerated with other grains by regions having a different composition (for example, a metal, alloy or amiorphous material) to the grains.
Methods described in the prior art for synthesising nano materials include gas phase synthesis, ball milling, co-precipitation, sol gel, and micro emulsion methods. The methods are typically applicable to different groups of materials, such as metals, alloys, intermetallics, oxides and non-oxides. A brief discussion of each will follow:
Gas-Phase Synthesis
Several methods exist for the synthesis of nano-particles in the gas phase. These include Gas Condensation Processing, Chemical Vapour Condensation, Microwave Plasma Processing and Combustion Flame Synthesis (H. Hahn, xe2x80x9cGas Phase Synthesis of Nanocrystalline Materialsxe2x80x9d, Nano Structured Materials, Vol 9, pp 3-12, 1997). In these methods the starting materials (suitable precursors to a metal, alloy or an inorganic material) are vaporised using energy sources such as Joule heated refractory crucibles, electron beam evaporation devices, sputtering sources, hot wall reactors, etc. Nano-sized clusters are then condensed from the vapour in the vicinity of the source by homogenous nucleation. The clusters are subsequently collected using a mechanical filter or a cold finger. These methods produce small amounts of non-agglomerated material, with a few tens of gram/hour quoted as a significant achievement in production rate.
Ball Milling
Mechanical attrition or ball milling is another method that can be used to produce nano-crystalline materials (C. C. Koch, xe2x80x9cSynthesis of Nanostructured Materials by Mechanical Milling: Problems and Opportunitiesxe2x80x9d, Nano Structured Materials, Vol 9, pp 13-22, 1997). Unlike the aforementioned methods, mechanical attrition produces the nano-materials not by cluster assembly but by the structural decomposition of coarser-grained materials as a result of severe plastic deformation. The quality of the final product is a function of the milling energy, time and temperature. To achieve grain sizes of a few nanometers in diameter requires relatively long processing times (several hours for small batches). Another main drawback of the method is that the milled material is prone to severe contamination from the milling media.
Co-Precipitation
In some special cases it is possible to produce nano-crystalline materials by precipitation or co-precipitation if reaction conditions and post-treatment conditions are carefully controlled (L. V. Interrante and M. J. Harnpden-Smith), Chemistry of Advanced Materialsxe2x80x94An Overview, Wileyxe2x80x94VCH (1998)). Precipitation reactions are among the most common and efficient types of chemical reactions used to produce inorganic materials at industrial scale. In a precipitation reaction, typically, two homogenous solutions are mixed and an insoluble substance (a solid) is subsequently formed. Conventionally, one solution is injected into a tank of the second solution in order to induce precipitation, however, simultaneous injection of the two solutions is also possible. The solid that forms (called the precipitate) can be recovered by methods such as filtration.
The precursor material has subsequently to be calcined in order to obtain the final phase pure material. This requires, in particular, avoidance of phenomena that induce segregation of species during processing such as partial melting for example. Formation of stable intermediates also has to be avoided since the transformation to the final phase pure material might become nearly impossible in that case. Typical results for surface areas for single oxides can be of several tens of m2/g. However, for a multi-cation compound, values less than 10 m2/g become more common.
Sol-gel Synthesis
Sol-gel synthesis is also a precipitation-based method. Particles or gels are formed by xe2x80x98hydrolysis-condensation reactionsxe2x80x99, which involve first hydrolysis of a precursor, followed by polymerisation of these hydrolysed percursors into particles or three-dimensional networks. By controlling the hydrolysis-condensation reactions, particles with very uniform size distributions can be precipitated. The disadvantages of sol-gel methods are that the precursors can be expensive, careful control of the hydrolysis-condensation reactions is required, and the reactions can be slow.
Microemulsion Methods
Microemulsion methods create nanometer-sized particles by confining inorganic reactions to nanometer-sized aqueous domains, that exist within an oil. These domains, called water-in-oil or inverse microemulsions, can be created using certain surfactant/water/oil combinations.
Nanometer-sized particles can be made by preparing two different inverse microemulsions (eg (a) and (b)). Each microemulsion has a specific reactant dissolved in the aqueous domains. The inverse microemulsions are mixed, and when the aqueous domains in (a) collide with those in (b), a reaction takes place that forms a particle. Since the reaction volumes are small, the resultant particles are also small. Some microemulsion techniques are reviewed in xe2x80x9cNanoparticle and Polymer Synthesis in Microemulsionxe2x80x9d, J. Eastoe and B. Warne, Current Opinion in Colloid and Interface Science, vol. 1 (1996), p800-805, and xe2x80x9cNanoscale Magnetic Particles: Synthesis, Structure and Dynamicsxe2x80x9d, ibid, vol. 1 (1996), p806-819.
A major problem with this technique is that the yield (wt product/wt solution) is small. Most microemulsion systems contain less than xcx9c20 vol % aqueous domains, which reduces the yield from the aqueous phase reactions by a factor of xcx9c5. Many of the aqueous phase reactions themselves already have low yields, therefore a further significant reduction in yield is very undesirable. The method also requires removal of particles from the oil, This can be very difficult for nanosised particles surrounded by surfactant, since these particles can remain suspended in solution, and are very difficult to filter due to their small size. Once the particles are separated, residual oil and surfactant still needs to be removed. Another serious disadvantage is that reaction times can be quite long. These aspects together would greatly increase the size, complexity and cost of any commercial production facility.
Use of Surfactants
Recently, there has been considerable research and development into the production of high surface area metal oxides using xe2x80x9csurfactant templatingxe2x80x9d. Surfactants are organic (carbon-based) molecules. The molecules have a hydrophilic (ie has an affinity for water) section and a hydrophobic (ie does not have an affinity for water) section.
Surfactants can form a variety of structures in aqueous (and other) solutions dependent upon the type of surfactant, the surfactant concentration, temperature, ionic species, etc. The simplest arrangement is individual surfactant molecules dispersed in solution. This typically occurs for very low concentration of surfactants. For higher concentrations of surfactant, the surfactant can coalesce to form xe2x80x9cmicellesxe2x80x9d. Micelles can be spherical or cylindrical. The diameter of the micelle is controlled mainly by the length of the surfactant chain and can range between xcx9c20 angstroms and xcx9c300 angstroms.
Even higher concentrations of surfactant give rise to more ordered structures called xe2x80x9cliquid crystalsxe2x80x9d. Liquid crystals consist of ordered micelles (eg micellar cubic, hexagonal) or ordered arrays of surfactant (eg lamella, bicontinuous cubic), within a solvent, usually water.
A paper published by C T Kresge, M E Leonowicz, W J Roth, J C Varruli and J S Beck, xe2x80x9cOrdered Mesoporous Molecular Sieves Synthesized by a Liquid Crystal Template Mechanismxe2x80x9d, Nature, vol 359 (1992) p710-712, described the production of inorganic materials having ordered porosity. In the process described in this paper, an ordered array of surfactant molecules was used to provide a xe2x80x9ctemplatexe2x80x9d for the formation of the inorganic material. The basic premise for this process was to use the surfactant structures as a framework and deposit inorganic material onto or around the surfactant structures. The surfactant is then removed (commonly by burning out or dissolution) to leave a porous network that mimics the original surfactant structure. The process is shown schematically in FIG. 1. Since the diameter of the surfactant micelles can be extremely small, the pore sizes that can be created using the method are also extremely small, and this leads to very high surface areas in the final product.
There are several characteristic features of the materials that have been produced using surfactant templating process as described above:
(a) An Ordered Pore Structure
As shown in FIG. 1, surfactant-templating methods use ordered surfactant structures to template deposition of inorganic material. The surfactant is then removed without destroying the ordered structure. This results in an ordered pore network, which mimics the surfactant structure.
The size of the pores, the spacing between pores, and the type of ordered pore pattern are dependent upon the type of surfactant, the concentration of the surfactant, temperature and other solution variables. Pores sizes between xcx9c20 angstroms and xcx9c300 angstroms have been achieved. Spacings between the pores also lie approximately within this range.
Periodic order at this scale can be detected using x-ray diffraction (XRD). In an XRD scan, signal intensity is plotted against the angle of the incident x-ray beam on the sample. Periodic structures give rise to peaks on XRD scans. The length of the periodic spacing is inversely related to the angle at which the peak occurs. Periodic arrangements of atorns (crystals), in which the spacings are very small, produce peaks at so-called xe2x80x98high anglesxe2x80x99 (typically greater than 5xc2x0). The ordered pore structures in surfactant-templated materials have much greater spacings, and therefore produce peaks at low angles (typically much less than 5xc2x0). A special XRD instrument, called a small angle x-ray scattering (SAXS) instrument, is commonly used to examine the pore structure in surfactant templated materials. An example of an XRD scan from a surfactant-templated material is shown in FIG. 2.
(b) Uniform Pore Size
For a given type of surfactant, surfactant micelles are essentially the same size. Pore sizes are therefore very uniform since pores are created in the space that was occupied by the micelles. Pore size distributions in materials may be obtained using nitrogen gas absorption instruments. An example of a pore size distribution from a surfactant-templated material is shown in FIG. 3. The distribution is extremely narrow, and is approximately cantered on the diameter of the surfactant micelles. Such distributions are typical for surfactant templated materials.
(c) Absence of atomic crystallinity (i.e. absence of highly ordered atomic structures).
Most conventional inorganic materials are crystalline. That is, their atoms are organised into highly ordered periodic structures. The type, amount and orientation of crystals in inorganic materials critically influences many important physical properties. A major drawback of most surfactant-templated materials is that normally the inorganic material is not highly crystalline. In fact in most cases it is considered amorphous.
The difficulties in producing highly crystalline materials derive from restrictions imposed by the very nature of surfactant templating. These restrictions greatly limit the types of reactions that can be used to form inorganic material. Obviously the inorganic material must form whilst the surfactant structure is preserved. Since the surfactant structure normally exists in an aqueous-based solution, the inorganic reactions must be aqueous-based, and must occur at temperatures less than 100xc2x0 C. This restriction is severe. Many conventional metal oxide materials, particularly complex multi-component oxides, require heat treatments at very high temperatures (up to 1200C.xc2x0) in order to achieve the correct crystal structure and a uniform dispersion of elements.
(c) Long reaction times
Most surfactant-templating methods require long reaction times to form the surfactant-inorganic structure. Following this, extended and careful heat treatment is usually necessary to remove the surfactant. Long reaction times greatly add to the expense and inconvenience of processing at a practical scale. The long reaction times again can be attributed to the types of inorganic reactions that must be employed in surfactant templating.
A variant on the surfactant templating method described above may be described as the production of surfactant-templated structures via self assembly. Many of the detailed mechanisms of this process are not clear, however the basic principle is that the surfactant-inorganic structures assemble at a substrate or a nucleus and grow from there. A general review of this method is given by Aksay-IA; Trau-M; Manne-S; Honma-I; Yao-N; Zhou-L; Fenter-P; Eisenberger-PM; Grune-SM xe2x80x9cBiomimetic pathways for assembling inorganic thin filmxe2x80x9d, Science vol. 273 (1996), p 892-898.
In self-assembly, the solution must be carefully controlled so that inorganic deposition only occurs on the assembling surfactant structure. If the inorganic phase forms too rapidly, then large inorganic precipitates that do not contain surfactant will form and drop out of solution. Clearly this would result in a non-porous structure.
The inorganic reactions that have mostly been employed in self-assembly (and other surfactant-templating methods as well) are called xe2x80x98hydrolysis-condensationxe2x80x99 reactions. Hydrolysis-condensation reactions involve an xe2x80x98inorganic precursorxe2x80x99, which is initially dissolved in solution. The first step in the reaction is hudrolysis of the precursor. This is followed by polymerisation of the hydrolysed precursor (condensation) to form an inorganic phase. Hydrolysis-condensation reactions may be represented generally as:
Mxe2x88x92OR+H2O Mxe2x88x92OH+ROH hydrolysis
Mxe2x88x92OH+Mxe2x88x92OR Mxe2x88x92Oxe2x88x92M+ROH condensation
M=a metal ion
R=an organic ligand, e.g. CH3 
Mxe2x88x92OR=inorganic precursor, commonly an alkoxide
The polymerisation nature of these reactions results in glass-like materials that do not contain a high degree of atomic order. As discussed previously this is a major limitation of most surfactant-templated materials. It is possible to increase the order in the inorganic material by heat treating at high temperatures, but almost all attempts to do this have resulted in collapse of the pore structure prior to crystallisation.
Most hydrolysis-condensation reactions are too rapid in aqueous solutions to be useful for surfactant templating. Silica-based reactions are an exception, and can be controlled very well. This explains why, for a long time, the only surfactant templated materials produced were either silica or silica-based.
Some success has been achieved with a number of other materials by using additives that slow down the hydrolysis condensation reactions in aqueous solutions. Examples are: xe2x80x9cSynthesis of Hexagonal Packed Mesoporous TiO2 by a Modified sol-gel Methodxe2x80x9d Agnew. Chem. Int. Edition English, vol. 34 (1995), p2014-2017, D. M. Antonelli and J. Y. Ying, ibid, vol. 35 (1996) p426, M. Froba, O. Muth and A. Reller, xe2x80x9cMesostructured TiO2: Ligand-stabilised Synthesis and Characterisationxe2x80x9d, Solid State Ionics, vols. 101-103 (1997), p249-253. A relevant patent is U.S. Pat. No. 5,958,367 (J. Y. Ying, D. M. Antonelli, T. Sun).
A major advance was accomplished by Stuckey et. al., (xe2x80x9cGeneralised Synthesis of Large-pore Mesoporous Metal Oxides with Semicrystalline Frame worksxe2x80x9d, P. Yang. D. Zhao, D. I. Margolese, B. F. Chmelka and G. D. Stucky, Nature, vol. 396 (1998), p152-155) who used alcohol-based solutions rather than aqueous solutions to form surfactant-templated structures. Hydrolosis-condensation reactions are much more easily controlled in alcohol solutions than aqueous solutions. Stucky et al. were therefore able to produce surfactant-templated structures with a range of inorganic metal oxides. Stucky, et. al. also reported that their materials exhibited some crystallinity in the organic phase. However the amount of crystallinity was still small, and the inorganic phase consisted of very small crystalline regions surrounded by amorphous inorganic material.
Surfactant-templated Structures via In-situ Reaction in Liquid Crystals
In this method, a solution of water and an organic precursor is mixed with an appropriate amount of surfactant, and this mixture is kept at a temperature where the surfactant organises to form a liquid crystal. The inorganic precursor then reacts to form inorganic material that occupies the space between the surfactant micelles. Finally the surfactant and any remaining water are removed by burning out or other methods.
Similar to the case for assembling surfactant structures, the inorganic reaction must take place while the surfactant structure is preserved. This again limits the temperature of the reaction, and the reaction must take place in an aqueous solution. Also, the reaction should not proceed prior to, or during, mixing with the surfactant.
The majority of research has used the same silicate hydrolysis-condensation reactions described in the self-assembly method. The liquid crystal structure is retained in the final product, as evidenced either by small angle XRD peaks or TEM. High angle XRD peaks, which would indicate atomic crystalline structures, are not present.
A different reaction method has been employed to produce cadmium sulfide, as outlined in xe2x80x9cSemiconducting Superlattices templated by Molecular Assembliesxe2x80x9d, P. Braum, P. Osenar and S. I. Stupp, Nature vol. 380 (1996) p325-327, and xe2x80x9cCountering Effects in Liquid Crystal Templating of Nanostructured CdSxe2x80x9d, V. Tohver et. al. Chemistry of Materials Vol 9, No. 7 (1997), p1495. Cadmium sulfate, cadmium chloride, cadmium perchlorate and cadmium nitrate aqueous solutions were mixed with surfactants to create liquid crystals. H2S gas was infused into the structure, which reacted with the dissolved cadmium ions to produce CdS. The liquid Crystal structure is retained in final product. Importantly, significant high-angle x-ray peaks are present indicating good atomic crystallinity.
Surfactant-templated Structures via Electrodeposition in Liquid Crystals
This method uses a similar principle to the surfactant-templating methods described above. An aqueous-based electroplating solution is mixed with surfactant at an appropriate concentration to form a liquid crystal. The mixture is placed between two electrodes, and kept at a temperature where the surfactant organises to form a liquid crystal. One of the electrodes is a substrate that is to be coated. Applying an appropriate voltage causes inorganic material to be deposited at one electrode. This material only deposits in the space between the surfactant. Upon completion of electrodeposition, the surfactant may be removed by heating or by dissolution in a solvent that does not attack the inorganic material.
The organised pore structure is maintained in this method. The deposited material is almost always metal, which is very easy to crystallise, therefore strong high-angle XRD peaks are observed. Platinum and tin have been produced by this technique.
As mentioned above, it is an aim of the surfactant-templating methods described above to produce solid material having a regular array of pores, with the pore structure having a very narrow pore size distribution (i.e. the pores are essentially of the same diameter). Most of the surfactant-templating processes described in the literature have resulted in the formation of inorganic particles having a particle size in excess of one micrometre. Crytallinity is difficult to obtain. Reaction times are lengthy because significant time is required to form the surfactant-inorganic structure in solution. Indeed, a number of published papers require time periods in the range of 1 day to 7 days to allow the desired surfactant-inorganic structure to develop. Furthermore, the conditions used to deposit the inorganic material in the surfactant structure must be xe2x80x9cgentlexe2x80x9d in order to avoid collapse of the surfactant structure.
Another approach to producing nanopowders is described in U.S. Pat. No. 5,698,483 to Ong et al. In this patent, a metal cation salt/polymer gel is formed by mixing an aqueous continuous phase with a hydrophilic organic polymeric disperse phase. When the hydrophilic organic polymer is added to the solution, the hydrophilic organic polymer absorbs the liquid on to its structure due to chemical affinity. The product is a gel with the metal salt solution xe2x80x9cfrozenxe2x80x9d within the dispersed polymeric network. The salt/polymer network is calcined to decompose the powder, leaving a high surface metal oxide powder. The calcining temperature is stated to be from 300xc2x0 C., preferably 450xc2x0 C.
This patent requires that a hydrophilic organic polymer be used in the process for making metal oxide powders.
Other patents that describe the production of nanometre-sized powders include U.S. Pat. No. 5,338,834 (incorporate a metal salt solution into a polymeric foam and calcining the foam to remove organics and leave a powder) and U.S. Pat. No. 5,093,289 (a foam matrix is coated with a suspension of silicon powder, synthetic resin and solvent and is subject to a heat treatment during which the foam is expelled and the silicon is stabilized).